feat: WebSocket support in relay — browsers connect directly, no bridge

Implements WS_RELAY_SPEC.md: relay handles both QUIC and WebSocket clients
in shared rooms, eliminating the wzp-web bridge server.

Room abstraction (room.rs):
- New ParticipantSender enum: Quic(transport) | WebSocket(mpsc::Sender)
- send_raw() sends PCM bytes to either transport type
- join_ws() convenience method for WS clients
- Forwarding loops handle mixed QUIC+WS rooms:
  QUIC→QUIC: send_media (trunked if enabled)
  QUIC→WS: send_raw payload bytes
  WS→QUIC: send_raw wraps in MediaPacket
  WS→WS: send_raw binary

WebSocket handler (ws.rs):
- GET /ws/{room} → WebSocket upgrade via axum
- Auth: first msg {"type":"auth","token":"..."} → validates against FC
- mpsc channel bridges room fan-out to WS binary frames
- Session + presence lifecycle matches QUIC path
- Optional static file serving via --static-dir (tower-http ServeDir)

Config: --ws-port 8080, --static-dir ./static
Proto: MediaHeader::default_pcm() for WS→QUIC wrapping

63 relay + 54 proto tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/Cargo.toml

@@ -25,7 +25,9 @@ serde_json = "1"
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
 quinn = { workspace = true }
 prometheus = "0.13"
-axum = { version = "0.7", default-features = false, features = ["tokio", "http1"] }
+axum = { version = "0.7", default-features = false, features = ["tokio", "http1", "ws"] }
+tower-http = { version = "0.6", features = ["fs"] }
+futures-util = "0.3"
 
 [[bin]]
 name = "wzp-relay"